Passports, visas, identity cards, driver licenses, bank cards, credit cards, security entrance cards, etc, must be made forgery-free to avoid fraudulent use. Therefore not only the finished document comprises security features, but also the paper on which such documents are printed comprises several security features. Such features are inclusions of materials in the bulk of the paper, e.g. watermarks, special relief patterns on the paper surface, fibres, security threads, light diffraction marks, etc. Paper including such security features does not present an even smooth surface for printing and therefore it is not very straightforward to print even density patches on security paper when using contact-printing methods.
Logically then, methods for printing on security paper are to be found in the field of non-impact printing. Among the most common methods of non-impact printing are electro(photo)graphy, ink-jet printing, thermosublimlation printing and Direct Electrostatic (DEP).
Thermosublimation printing is not well suited for printing on rough surfaces and mostly a dye acceptor layer is necessary on the substrate. Thermosublimation printing, that proceeds by thermally evaporating solid dye or pigments, is not very well suited for security printing because of the dyes, usually used, are not sufficiently waterfast and lightfast, and are characterize by high bleeding, leading to documents with a restricted shelf life. Thermosublimation printing does thus not offer an adequate possibility for printing on security paper.
Ink-jet printing offers at first sight interesting possibilities for printing on paper with a very rough surface, but is not very well suited for printing security documents. The dyes or pigments, usually used in ink-jet printing, are not sufficiently waterfast and lightfast to be used in security documents. Moreover, in ink-jet printing also, an ink-receiving layer is necessary on the substrate.
Therefore electro(photo)graphic and Direct Electrostatic Printing are preferred non-impact printing methods for security printing. The advantage of these methods is that they use pigmented and/or dyed toner particles that are fused to the substrate, and that in the preparation of said toner particles the chemical structure of the pigments or dyes (chemical structure defining largely the water- and lightfastness) that are used is not very critical. Thus in the production of toner particles a wide range of different pigments and dyes can be used. It is, e.g., possible to incorporate nacreous, iridescent or interference pigments, etc, in the toner particles, without interfering with the usefulness of this toner particles in the printing methods.
In electro(photo)graphy an image is first formed on a latent image bearing member and then transferred to a substrate, which in this case is a security paper with a rough surface. The transferring step is still a contact step and therefore the image of even density patches on the substrate, having a rough substrate or comprising watermarks, is not very faithful.
From the printing methods using toner particles to form an image, DEP has the advantage to be a real non-impact method and is therefore the preferred method for printing on security paper having a rough surface.
The DEP method is a well known printing method, a DEP printing device is disclosed in e.g. U.S. Pat. No. 3,689,935. This document discloses an electrostatic line printer having a multi-layered particle modulator or printhead structure comprising:
a layer of insulating material, called isolation layer; PA1 a shield electrode consisting of a continuous layer of conductive material on one side of the isolation layer; PA1 a plurality of control electrodes formed by a segmented layer of conductive material on the other side of the isolation layer; and PA1 at least one row of apertures. PA1 presenting an uniform cloud of toning particles to the printhead. PA1 supplying sufficient charged toning particles to the printhead structure, without scattering them or without contaminating the printhead structure and the engine environment. PA1 prevent clogging of the apertures in the printhead structure PA1 avoiding the deposition of wrong sign particles on the printhead structure, the repelling state for particles with the right sign corresponds to an attraction state for wrong sign particles. PA1 means (V4 and V1) for providing a DC potential difference (DC.sub.TB) between a back electrode (105) and means for delivering toner particles (101) and for creating a flow of toner particles (104) from said means for delivering toner particles towards said back electrode, PA1 a printhead structure (106), having printing apertures (107) and control electrodes (106a) around said printing apertures, interposed between said means for delivering toner particles to said back electrode and being spaced from said back electrode by a spacing having an adjustable width L in .mu.m, for image wise controlling said flow of toner particles, PA1 means (108) for passing a substrate, having thickness T in .mu.m, in said spacing L between said back electrode and said printhead structure, PA1 means (111) for adjusting said width L in such a way that L.ltoreq.1.5 T, and PA1 means (110) for fixing said toner particles to said substrate.
Each control electrode is formed around one aperture and is isolated from each other control electrode.
Selected potentials are applied to each of the control electrodes while a fixed potential is applied to the shield electrode. An overall applied propulsion field between a toner delivery means (toner source) and a receiving member support projects charged toner particles through a row of apertures of the printhead structure. The intensity of the particle stream is modulated according to the pattern of potentials applied to the control electrodes. The modulated stream of charged particles impinges upon a receiving member substrate, interposed in the modulated particle stream. The receiving member substrate is transported in a direction orthogonal to the printhead structure, to provide a line-by-line scan printing. The shield electrode may face the toner delivery means (toner source) and the control electrode may face the receiving member substrate. A DC field is applied between the printhead structure and a single back electrode on the receiving member support. This propulsion field is responsible for the attraction of toner to the receiving member substrate that is placed between the printhead structure and the back electrode. Due to the electrical nature of said imaging process, accurate control of the distance of said printhead structure to said toner application module and said image receiving layer is very important.
Several modifications on the basic design of U.S. Pat. No. 3,689,935 have been proposed, mainly to overcome four major problems:
Typical examples of modification are disclosed in, e.g., GB 2,108,432, DE-OS 3,411,948, U.S. Pat. No. 4,780,733, U.S. Pat. No. 4,814,796, U.S. Pat. No. 4,912,489, U.S. Pat. No. 5,038,159, U.S. Pat. No. 5,036,341, U.S. Pat. No. 5,121,144, U.S. Pat. No. 5,170,185, U.S. Pat. No. 5,281,982 and many others. In these disclosure the toner particles are presented in the vicinity of a printhead structure via a toner delivery means bringing only toner particles in the vicinity of the printhead structure. In U.S. Pat. No. 5,327,169, EP-A 675 417 and JP-A 60/263962, is disclosed wherein developer, i.e. not only toner particles but also carrier particles, is presented in the vicinity of the printhead structure directly with a magnetic brush.
All the DEP devices, disclosed in the references above, have in common that between the surface of the printhead structure, facing the back electrode and the surface of the back electrode an air gap exists through which the substrate can pass and the toner particles fly over this air gap from the printhead structure on to said substrate. Most of the disclosures remain silent over the width of the air gap. In EP-A 675 417 a gap of 150 .mu.m is disclosed, in U.S. Pat. No. 5,257,046 a gap between 127 .mu.m and 762 .mu.m is disclosed and in JP-A 60/263962 a gap of 600 .mu.m is disclosed.
For printing on security paper, that is paper comprising various security features and that a.o. can comprise watermarks and/or can be both quite thick and have large thickness variations in the order of tens and even hundreds of .mu.m, the DEP devices disclosed earlier do not give optimum results.